Fix <0 error with paging

Author: newsch

src/fe_modes.c

@@ -160,26 +160,35 @@ int master_handler(State *state, WINDOW *canvas_win, WINDOW *status_win) {
     // shift view down/up/left/right
     // renaming for ease of use
     const int h = getmaxy(canvas_win) - 2;  // height of visible canvas
-    const int w = getmaxx(canvas_win) - 2;  // width
+    const int w = getmaxx(canvas_win) - 2;  // width of visible canvas
     const int vy = state->view->y;
     const int vx = state->view->x;
-    const int ch = state->view->canvas->num_rows;
-    const int cw = state->view->canvas->num_cols;
+    const int ch = state->view->canvas->num_rows;  // height of total canvas
+    const int cw = state->view->canvas->num_cols;  // width of total canvas
     int new_vy = vy;
     int new_vx = vx;
     // calc shift based on key
     switch (c) {
       case KEY_NPAGE:  // down
-        new_vy = min(vy + h, ch - h);
+        // b/c canvas snaps to the top left corner, we need to be more careful
+        // when moving down or right. Move to either:
+        //           another window height
+        //           |       or the end of the visible canvas
+        //           |       | (when the canvas is smaller than the window, this
+        //           v       v is less than zero, so get the max of the two)
+        new_vy = min(vy + h, max(ch - h, 0));
         break;
       case KEY_PPAGE:  // up
         new_vy = max(0, vy - h);
         break;
-      case KEY_SRIGHT:
-        new_vx = min(vx + w, cw - w);
+      case KEY_SRIGHT:  // right
+        // similar situation to moving down - jump to another window width or
+        // the end of the canvas, and check for negative values
+        new_vx = min(vx + w, max(cw - w, 0));
         break;
-      case KEY_SLEFT:
+      case KEY_SLEFT:  // left
         new_vx = max(0, vx - w);
+        break;
     }
     // shift view
     state->view->y = new_vy;